1. Field of the Invention
The present invention relates to a radiographic apparatus for detecting, with an image detector, intensity distribution of radiation such as X-rays transmitted through an object such as a human organism.
2. Description of the Related Art
As a first method for obtaining a radiation image of an object, a so-called film/screen method is known. According to this method, which is most generally used, fluorescent screens, which are made from rare earth and radiate upon exposure with an X-ray, are placed and held in close contact with both surfaces of a radiographic sensitive film. The X-ray transmitted through an object is converted into a visual light ray by a phosphor, and the visual light ray in turn forms a latent image on the radiographic sensitive film so as to be developed and visualized by developing means.
A second method for obtaining a radiation image of an object, a so-called computed radiography (CR) method, also has been put to practical use. According to the computed radiography method, the intensity distribution of radiation (radiation image) transmitted through an object is temporarily stored within a phosphor as a latent image, and then, by irradiating exciting light onto the phosphor, the latent image is read out. For example, when being irradiated by radiation such as X-rays, α-rays, β-rays, γ-rays, electron rays, and ultra-violet rays, a type of phosphor stores part of energy of the radiation, and thereafter, when being irradiated by exciting light such as visible light, it radiates stimulated emission corresponding to the stored energy.
A radiation-image-information recording and reproducing system using such a phosphor called a cumulative phosphor or photostimulable phosphor is disclosed in Japanese Patent Laid-Open No. 55-12429 and Japanese Patent Laid-Open No. 56-11395. In such systems, the radiation image information of an object is temporarily recorded on a cumulative phosphor sheet, which in turn is scanned using exciting light such as laser light so as to produce stimulated emission light; an image signal is obtained by photoelectrically reading the obtained stimulated emission light, so that the radiation image of the object is visualized on the basis of the obtained image signal by utilizing a recording apparatus using a recording material such as a photosensitive material or utilizing a display such as a CRT.
A third method for obtaining a radiation image of an object uses a semiconductor sensor (also called as a semiconductor image detector or solid image pick-up elements) developed along with a recent technical progress in a semiconductor manufacturing process. In radiographic systems using the semiconductor sensor, the intensity distribution of radiation transmitted through an object is read out by photoelectric converting means with a wide dynamic range so as to convert it into an electric signal. Then, based on this electric signal, a radiation image is output as a visible image using a recording apparatus or display. Since the semiconductor sensor has a wide dynamic range so as to be able to detect a wide range amount of radiation, a radiographic system using the semiconductor sensor is much less likely to be affected by variations of the radiation exposure amount, enabling an excellent radiation image always to be obtained. This type of system has an advantage of being able to detect radiation images having the intensity distribution with an extremely wide range in comparison with systems using the film/screen method described above.
FIG. 11 shows the radiographic system using the semiconductor sensor described above as an example of a conventional radiographic system. A radiographic apparatus 1 having an examinee S placed thereon houses a radiographic image detector 2 having a detection surface with plural photoelectric conversion elements formed in a matrix arrangement. X-rays irradiated from an X-ray generator 3 are detected by the radiographic image detector 2 after transmitting through the examinee S. The image signal output from the radiographic image detector 2 is displayed on a monitor (display) 5 as a visualized X-ray image of the examinee S after digital image processing is performed thereon by image processing means 4. In addition, while X-rays have been described as an example of radiation, the radiation is not necessarily limited to X-rays.
When using any one of three methods described above, the radiographic system may have a grid, arranged between an examinee S and an X-ray receiver, for eliminating scattering X-rays produced inside the examinee S so as to improve contrast of X-ray images. The grid comprises plural sheets of foil with large X-ray absorptance, intermediate materials with small X-ray absorptance, the sheets and the materials being arrayed alternately in a stripe arrangement, and a cover member with large X-ray transmittance for covering the foil and intermediate materials.
The grid is arranged between the examinee S and the radiographic image detector 2 as shown in FIG. 11, for example. A primary X-ray, transmitted through the examinee S without being scattered by the examinee S, enters the intermediate material in the direction substantially along the foil surface of the grid, i.e., substantially parallel to the foil, so as to transmit through the intermediate material. Whereas a secondary X-ray scattered by the examinee S, i.e., a scattered X-ray, comes at an angle, not in parallel, with respect to the foil surface, so that a large portion thereof impinges upon the foil to be absorbed thereby. Therefore, the grid prevents the resolution of the radiographic system from degrading due to the scattered X-ray.
FIG. 12 shows a conventional radiographic apparatus using the first method described above and the grid, wherein a so-called bookie unit 12 is assembled in a vertically movable radiographic base 11 as a radiographic image detector. The radiographic base 11 comprises a base 13, in which the upper surface is vertically movable, and a cover 14 divided for covering side surfaces of the base 13. A top board 17 is supported on the upper surface of the base 13 movably in front-and-back and right-and-left directions via a pair of rails 15 and a pair of rails 16. The bookie unit 12 is supported on the upper surface of the base 13 movably in the right-and-left direction via a pair of rails 18.
The bookie unit 12 comprises a cassette detachably housed therein as an X-ray receiver for containing a film and phosphor therein, a so-called bookie part housed therein for constituting a grid held movably in parallel for eliminating scattered X-rays, and a cover 12a made of a material with high X-ray transmittancy and arranged in the upper part thereof.
As an examinee is lying on the top board 17, when the weight of the examinee is 100 kg, for example, the top board 17 deflects downwardly by 10 to 20 mm. Accordingly, it is required to have a clearance more greater than the deflection between the top board 17 and the bookie unit 12. In general, increasing the clearance between an examinee and the X-ray receiver (the bookie unit 12) causes the obtained X-ray image to become blurred so that the resolving power (MTF) is reduced. Therefore, it is not preferable to increase the clearance.
Also, even when the bookie unit 12 is moved in the left utmost while the top board 17 is simultaneously moved in the right utmost, the left end of a film within the bookie unit 12 cannot be positioned in the vicinity of the left end of the top board 17, so that there is a problem to make a film of an end part of the examinee that is lying down. That is, while a film can easily be made by the bookie unit 12 of the part of the examinee ranging from the cervical spine to thigh, the bookie unit 12 cannot easily make a film of the end parts (the upper end or lower end) or extremities of the examinee, such as the head and lower limbs.
In such a case, instead of using the bookie unit 12, it is necessary to arrange a film cassette on the top board 17 so as to make the examinee be recumbent thereon. Moreover, it may be burdensome for an operator or examinee to properly arrange or to finely adjust the film cassette position relative to the recumbent examinee. For example, the operator may have to do the heavy work of lifting a handicapped examinee, or the examinee may experience pain.